Add update method GDSStore

[jakirkham]

In PR ( #131 ), KvikIO's GDSStore implemented getitems (effectively a vectored IO gather API). It would be nice to have a scatter implementation (to allow multiple writes at once)

Fortunately Zarr's BaseStore already inherits from MutableMapping, which supplies an update method (analogous to dict's update method) that can already take multiple items to add to a collection

So for KvikIO's GDSStore to add a scatter implementation would be as simple as overriding the update method in this class